/* clusterrank to control the subgraphs */
digraph "cr"
{
 yydebug=true;
 /* clusterrank=global; */
 /* clusterrank=none; is same as global */
 clusterrank=local; /* give cluster separate layout */
 10;
 subgraph cluster_1
 {
   a1->a2->a3;
 }
 subgraph cluster_2
 {
   b1->b2->b3;
 }
 10->a1;
 10->a3;
 a3->b3;
 /* now the same but without cluster */
 20;
 subgraph sg_1
 {
   aa1->aa2->aa3;
 }
 subgraph sg_2
 {
   bb1->bb2->bb3;
 }
 20->aa1;
 20->aa3;
 aa3->bb3;
}
